Incandescent lamp



April 5, 1960 c. F. ROBINSON 2,931,933

INCANDESCENT LAMP Filed March 28, 1958 INVENTOR. CHAR! 5.5 f RAB/N60 gJae/2% United States Patent ()fifice INCANDESCENT LAMP Charles F.Robinson, Pasadena, Calif., assignor to Bell & Howell Company, Chicago,111., a corporation of Illinois Application March 28, 1958, Serial No.724,693 4 Claims. (Cl. 313-113) The present invention relates to lampsand more particularly to an incandescent lamp.

The surface brightness of an optical radiating source such as anincandescent lamp is determined largely by the temperature of thefilament of the lamp and is comparatively independent of the nature ofthe material used in the filament. In order to obtain an incandescentlamp which will operate as a high-intensity source of optical radiation,it is necessary to utilize a material as the filament thereof which canbe operated at high temperatures. The material generally used for suchfilaments is tungsten because of its high-melting point (3,370centigrade). A filament of tungsten may be operated up to a temperatureof approximately 2700 centigrade as a simple self-heated helix. Whensuch a tungsten filament is operated for a considerable time at suchhigh temperatures, failure generally occurs as a result of evaporationof the tungsten reducing the cross-section at some point in the helix tosuch an extent that excessive heating occurs at that point which resultsin a burn-out or breaking of the helix. If it is attempted to circumventthis difliculty by using helices of thicker cross-section, failure ofthelamp then occurs primarily by the deposition of the evaporatedmaterial from the filament onto the window of the lamp envelope whichreduces the transparency of the window to such an extent that theintensity of the light transmitted through the window becomes too low tobe useful.

Certain other materials such as tantalum carbide, hafnium carbide andzirconium carbide have appreciably higher melting points than tungstenand can also be utilized as the filaments for incandescent lamps toobtain a higher surface brightness than can be achieved with a tungstenfilament. However, the boiling points of these materials are lower thanthat of tungsten so that the useful life of a lamp employing suchmaterials for a filament is limited to a great extent by evaporation ofthe material from the filament onto the window of the lamp envelope.

In accordance with the present invention an incandescent lamp for use asan optical radiating source is provided which overcomes the abovedisadvantages by preventing direct evaporation of the material from thefilament of the lamp onto the window or transparent portion of the lampenvelope. A lamp in accordance with the present invention includes afilament composed of a reasonable specular metal or other specularmaterial disposed within an envelope. A portion of the envelope istransparent to light and functions as a window. Reflecting means aresecured to the envelope for reflecting the light emitted from thefilament to the transparent portion or window of the envelope and ashield means is disposed within the envelope between the filament andthe transparent portion thereof for preventing direct evaporation of thematerial in the filament onto the transparent portion. Since thefilament is composed of a reasonably good specular material thedeposition of it onto the reflecting means has little effect on theoperation of 2,931,933 iatented Apr. 5, 1960 the lamp since it in turnacts as a reflector when so deposited. By preventing appreciable amountsof the material that is evaporated from the filament from depositingonto the transparent portion or window of the envelope the life of thelamp is greatly increased. There is no intrinsic limit on the amount ofmaterial which may be evaporated from the filament of such a lampwithout reducing the apparent brightness of the lamp.

The invention is described in more detail in reference to theaccompanying drawing which is a sectional elevation of one embodiment ofa lamp employing the principles of this invention.

Referring now to the drawing, there is shown an'incandescent lamp whichincludes an envelope 10 having a transparent po'rtion or window 11 atone side thereof. The envelope It? may be evacuated or filled with aninert gas as'is well known in the art. A filament 12 is disposed withinthe envelope adjacent the window 11. A pair of conductors 13 and 14- areconnected to the filament 12 at one end and extend through a baseportion 15 of the envelope 10. The conductors 13 and 14 are hermeticallysealed to the base portion 15 in a well known manner.- To energize thefilament 12 a suitable source of energizing potential (not shown) may beconnected to the ends of the conductors that are disposed outside of theenvelope 10. A light reflecting coating 16 is disposed on the innersurface of the top portion of the envelope 1!) which is diametricallyopposed to the window 11. The coating 16 and the portion of the envelopeon which it is disposed are arranged to reflect the light emitted fromthe filament 12 onto the window 11 or to project the image of thefilament 12 onto the window 11 as may be seen in the drawing, the imageof the filament being designated by the reference numeral 12 A shieldplate or baffle means 17 which is formed integrally with the envelope 10is disposed between the filament 12 and the Window 11 to block the lineof sight between these two elements of the lamp so that material willnot be evaporated from the filament 12 directly onto the window 11.

The filament 12 should be composed of a reasonably good specular metalor other material so that the material that is evaporated from thefilament and deposited onto the coating 16 during the operation of thelamp will itself act as a reflector. If the lamp is to be operated athigh temperatures to provide a high surface brightness of the filament12, the filament should also be composed of. a material that has arelatively high melting point. There are many materials that fulfillboth criteria, i.e., tungsten, tungsten carbide, tantalum, tantalumcarbide, hafnium carbide, zirconium carbide and rhenium. Where tungstenor tungsten carbide is utilized as a filament it is necessary to preventanyappreciable amount of oxvgen from coming into contact with thefilament by maintaining a very high vacuum or an atmosphere of inert gaswithin the envelope because tungsten readily oxidizes and the oxide oftungsten does not depo'sit to form a good reflector. Preferably thefilament is formed of rhenium which has a melting point slightly lowerthan the melting point of tungsten and is a very good specular metal.

There has thus been disclosed an incandescent lamp which may be operatedat very high temperatures for long periods of time without noticeablechange in the apparent brightness thereof.

I claim:

1. An incandescent lamp comprising an evacuated envelope having atransparent portion in a first side thereof, a filament composed of ahigh melting point specular material disposed within the envelopeadjacent the transparent portion, a light reflecting coating disposed ona portion of the inner surface of asecond side of the envelope that isdiametrically opposed to the first side, the

light reflecting coating being arranged to reflect light emitted fromthe filament to the transparent portion, and a plate secured to theenvelope adjacent the transparent portion thereof and extending into theinterior of the envelope between the filament and the transparentportion for preventing material evaporated from the filament fromdepositing directly onto the transparent portion, the light reflectingcoating being exposed directly to the filament whereby materialevaporated from the filament during operation of the lamp will bedeposited onto the light reflecting coating, the material from thefilament deposited on the coating being arranged to act as a refiectingcoating.

2. An incandescent lamp as defined in claim 1 wherein the filament iscomposed of rhenium.

3. An incandescent lamp as defined in claim 1 wherein the plate isformed integrally with the envelope.

4 4. An incandescent lamp as defined in claim 1 wherein the filamentiscornposed of a material chosen from the group of tungsten carbide,tantalum, tantalum carbide, hafnium carbide and zirconium carbide.

References Cited in the file of this patent UNTTED STATES PATENTS2,494,916 Van Horn Jan. 17, 1950 2,592,102 Alexander Apr. 8, .1952

2,699,515 Williams Jan. 11, 1955 2,822,495 Albright Feb. 4, 1 958FOREIGN PATENTS 762,566 Germany a Feb. 22, 1954

